The equation you use to size a relief valve or calculate the capacity of a relief valve will depend upon whether the flow is all vapor, all liquid, or 2-phase. Most relief valve manufacturers provide a technical reference manual that provides the flow of vapor and/or liquid through their relief valves. For critical flow through a relief valve, see also the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Appendix 11. An excellent reference for almost all questions on relief valves, refer to Guidelines for "Pressure Relief and Effluent Handling Systems," a CCPS book published in 1998 by the American Institute of Chemical Engineers. You can also see information provided on the DIERS web site:
web site by the company that voluntarily maintains DIERS web site). DIERS is the Design Institue for Emergency Relief Systems - a working subgroup that is part of the American Institute of Chemcial Engineers (since the 1980's).
